The development of biological control agents as alternatives to chemical insecticides for the control of important pest species is a subject of increasing interest. Concerns for the environment and exposure of man to harmful substances in air, food and water have stimulated legislation and restrictions regarding the use of chemical pesticides, particularly for pests found in the urban environment. Control of insect pests in urban areas is highly desirable but exposure to chemical pesticides in the household and from lawns and gardens is of great concern to the public. If given a choice, most people would prefer to use a non-toxic biological control agent rather than a toxic chemical to control insects in the urban environment. The problem is that very few biological alternatives to chemical insecticides are available for purchase and use by the public.
The soil microbe Bacillus thuringiensis (B.t.) is a Gram-positive, spore-forming bacterium characterized by parasporal crystalline protein inclusions. These inclusions often appear microscopically as distinctively shaped crystals. The proteins can be highly toxic to pests and specific in their toxic activity. Certain B.t. toxin genes have been isolated and sequenced, and recombinant DNA-based B.t. products have been produced and approved for use. In addition, with the use of genetic engineering techniques, new approaches for delivering B.t. endotoxins to agricultural environments are under development, including the use of plants genetically engineered with endotoxin genes for insect resistance and the use of stabilized intact microbial cells as B.t. endotoxin delivery vehicles (Gaertner, F. H., L. Kim [1988] TIBTECH6:S4-S7). Thus, isolated B.t. endotoxin genes are becoming commercially valuable.
Until the last ten years, commercial use of B.t. pesticides has been largely restricted to a narrow range of lepidopteran (caterpillar) pests. Preparations of the spores and crystals of B. thuringiensis subsp. kurstaki have been used for many years as commercial insecticides for lepidopteran pests. For example, B. thuringiensis var. kurstaki HD-1 produces a crystal called a .delta.-endotoxin which is toxic to the larvae of a number of lepidopteran insects.
In recent years, however, investigators have discovered B.t. pesticides with specificities for a much broader range of pests. For example, other species of B.t., namely israelensis and tenebrionis (a.k.a. B.t. M-7, a.k.a. B.t. san diego), have been used commercially to control insects of the orders Diptera and Coleoptera, respectively (Gaertner, F. H. [1989] "Cellular Delivery Systems for Insecticidal Proteins: Living and Non-Living Microorganisms," in Controlled Delivery of Crop Protection Agents, R. M. Wilkins, ed., Taylor and Francis, New York and London, 1990, pp. 245-255). See also Couch, T. L. (1980) "Mosquito Pathogenicity of Bacillus thuringiensis var. israelensis," Developments in Industrial Microbiology 22:61-76; Beegle, C. C., (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Krieg, A., A. M. Huger, G. A. Langenbruch, W. Schnetter (1983) Z. ang. Ent. 96:500-508, describe Bacillus thuringiensis var. tenebrionis, which is reportedly active against two beetles in the order Coleoptera. These are the Colorado potato beetle, Leptinotarsa decemlineata, and Agelastica alni.
Recently, new subspecies of B.t. have been identified, and genes responsible for active .delta.-endotoxin proteins have been isolated (Hofte, H., H. R. Whiteley [1989] Microbiological Reviews 52(2):242-255). Hofte and Whiteley classified B.t. crystal protein genes into 4 major classes. The classes were CryI (Lepidoptera-specific), CryII (Lepidoptera- and Diptera-specific), CryIII (Coleoptera-specific), and CryIV (Diptera-specific). The discovery of strains specifically toxic to other pests has been reported. (Feitelson, J. S., J. Payne, L. Kim [1992] Bio/Technology 10:271-275).
The cloning and expression of a B.t. crystal protein gene in Escherichia coli has been described in the published literature (Schnepf, H. E., H. R. Whitely [1981] Proc. Natl. Acad. Sci. USA 78:2893-2897). U.S. Pat. No. 4,448,885 and U.S. Pat. No. 4,467,036 both disclose the expression of B.t. crystal protein in E. coli. U.S. Pat. Nos. 4,797,276 and 4,853,331 disclose B. thuringiensis strain san diego (a.k.a. B.t. tenebrionis, a.k.a. M-7) which can be used to control coleopteran pests in various environments. U.S. Pat. No. 4,918,006 discloses B.t. toxins having activity against Dipterans. U.S. Pat. No. 4,849,217 discloses B.t. isolates which have activity against the alfalfa weevil. U.S. Pat. No. 5,208,077 discloses coleopteran-active Bacillus thuringiensis isolates. U.S. Pat. No. 5,151,363 and U.S. Pat. No. 4,948,734 disclose certain isolates of B.t. which have activity against nematodes. As a result of extensive research and investment of resources, other patents have issued for new B.t. isolates and new uses of B.t. isolates. However, the discovery of new B.t. isolates and new uses of known B.t. isolates remains an empirical, unpredictable art.
Ants comprise a large group of insects (family Formicidae) from the taxonomic order, Hymenoptera. They are among the most common house pests. In many situations, ants are a nuisance pest. Foraging ants create problems with hygiene in hospitals and the food industry. Ants also create problems in agriculture. Damage can be caused by direct feeding on plants. Harvester and fire ants are commonly associated with this type of damage (Holldobler, B., E. O. Wilson [1990] The Ants, Belkap Press, Cambridge, Mass. 732 pp.) Some ants cause indirect damage by nurturing and protecting sap feeding insects such as mealybugs and aphids. Ants, particularly in the genus Solenopsis are capable of producing extremely painful stings to humans. It has been estimated that approximately 10,000 stings occur each year (Habermehl, G. G. [1981] Venomous Animals and Their Toxins, Springer-Verlag, N.Y., 195 pp.). The pharaoh ant (Monomorium pharaonis) is primarily an urban pest. However, this species can also be an agricultural pest and damage to corn has been noted (Ebeling, W. [1978] Urban Entomology, UC Press, Berkeley, Calif., 695 pp.).
Carpenter ants, Camponotus spp., are distributed throughout North America. Some of the more common and/or studied species include C. modoc in the Pacific Northwest, C. clarithorax in southern California, and the black, red, and Florida carpenter ants, C. pennsylvanicus, C. noveboracensis and C. abdominalis, respectively, in the east (Ebeling, W. [1978] Urban Entomology, Univ. Calif.: Berkeley p. 209-213). Public concern over carpenter ants has been increasing due to the greater probability of structural infestations as suburban developments extend into the forest habitats of the ants.
Pestiferous species of carpenter ants may be considered nuisance pests because of their foraging activity inside homes. More significant damage occurs when carpenter ants extend their nests into sound wood. Nesting sites may be located in live and dead trees, sometimes resulting in damage to shade trees. Nests may also be established in walls and support beams of structures, or in voids within doors, walls, and furniture. Preference for moist or decaying wood has been reported, but nesting sites are not restricted to such areas. Carpenter ant populations develop relatively slowly with colonies of 300-2,000 workers being produced over a 2-year or longer period for various species. The presence of reproductives follows this slow development since their production has been reported only from well established colonies (Hansen, L. D., R. D. Akre [1985] Biology of carpenter ants in Washington state (Hymenoptera:Formicidae:Camponotus). Melanderia43. 62 p.; Pricer, J. L. [1908] Biol. Bull. 14:177-218). Despite the slow colony growth, large colonies with satellite colonies have been found. Worker movement occurs between the main colony and the satellites, which serve as areas for further brood development and colony expansion (Hansen and Akre [1985], supra).
Current methods for controlling structural infestations of carpenter ants include sanitation of potential and current nest sites, minimizing access to structures (eg. preventing the contact of tree branches with a structure), and the application of insecticides to repel (perimeter spray barriers) and/or eliminate carpenter ants. The use of boric acid dust in dry, wall voids is reported to be effective for up to 20 years (Hansen and Akre, supra).
Recommendations for the chemical control of established structural infestations in the home are often accompanied with warnings of possible hazards to the applicator as well as children and pets. Alternative control methods such as effective biological control agents have not been found (Akre, R. D., L. D. Hansen, A. L. Antonelli [1989] Ext. Bull. Washington State Univ. Coop. Ext. Serv. 1989 rev. no. EB 0818, 6 pp.). A need clearly exists for a safe, effective biological control agent for carpenter ants.
Pharaoh ants, Monomorium pharaonis, have been described as " . . . the most persistent and difficult of all our house-infesting ants to control or eradicate" (Smith, M. R. [1965] USDA-ARS Tech. Bull. No. 1326, 105 pp.). It is a tropical species which has extended its range to more temperate regions by establishing colonies in heated buildings. Pharaoh ants frequently infests buildings where food is prepared, and have been found to carry pathogenic organisms (Beatson, S. H. [1972] Lancet 1:425-427).
The difficulty in controlling pharaoh ants may be attributed to their inaccessible nesting sites, rapid population growth, and dispersion of colonies. Their small size allows establishment of colonies in any suitable location, including unusual places such as between books and in stored clothing. With multiple queen colonies, and the warm (30.degree. C.), humid (63-80% RH) conditions that favor pharaoh ants, large colonies can develop rapidly. Portions of these large colonies may disperse to form new colonies at any time, probably in response to overcrowding and unfavorable microenvironmental conditions. Unlike other ant species, pharaoh ants do not exhibit intercolony aggression. This permits the adoption of ants from other colonies and may further enhance the establishment of new colonies and reinfestations. Pharaoh ants also forage for food more than 35 m from the nest without distinct trail following, and thus make nests difficult to find and eradicate.
Control methods for pharaoh ants emphasize the use of insect growth regulators (IGR) or toxicants incorporated into baits. Properly implemented bait programs are effective, however it may take over a month to achieve control. Insecticide applications, while fast acting, usually do not eliminate colonies, and may be unacceptable in certain areas where toxic residues are a concern. In addition, insecticide applications are generally not compatible with bait programs.
A need exists for safe and effective biological control agents for pharaoh ants.